During the combustion process, gasoline is oxidized, and hydrogen (H) and carbon (C) combine with air. Various chemical compounds are formed including carbon dioxide (CO2), water (H2O), carbon monoxide (CO), nitrogen oxides (NOx), unburned hydrocarbons (HC), sulfur oxides (SOx), and other compounds.
Automobile exhaust systems include a three-way catalytic converter that helps oxidize CO, HC and reduce NOx in the exhaust gas. The catalytic converter includes an oxygen storage capability to provide a buffer for lean to rich air-to-fuel (AFR) deviations. For example, oxygen is stored in the catalytic converter during lean operation (i.e., excess air) and is depleted from the catalytic converter during rich operation (i.e., excess fuel).
During idle, engines may be operated using a lean AFR (i.e., an AFR greater than stoichiometry (AFRSTOICH)) to improve fuel consumption. More specifically, because a lean AFR is used, less fuel is consumed during idle. However, extended lean operation presents a challenge for exhaust after-treatment. One challenge is that the catalytic converter's NOx conversion efficiency falls off rapidly as the AFR goes lean and the catalyst becomes saturated with oxygen. Lean NOx trapping after-treatment technology has been developed to address this issue.
Another challenge is that excess oxygen is stored in the catalytic converter. More specifically, catalytic converters are formulated to store a targeted mass of oxygen. This enhances catalyst efficiency by acting as a buffer for small rich deviations, during which oxygen is released for oxidation, and lean deviations, during which the excess oxygen is stored. During extended lean operation, the catalytic converter becomes saturated with oxygen. The NOx conversion efficiency is then reduced until some of the excess oxygen is removed. The excess oxygen must be removed prior to returning to stoichiometric operation (i.e., operation using AFRSTOICH), for proper 3-way (i.e., HC, CO, and NOx) conversion efficiency to resume.
Engine control systems can remove the excess oxygen with a short period of rich operation after lean idle. As a result, excess fuel is consumed. This fuel consumption penalty cancels out some of the benefit of lean idle operation.